🧮 Beginner’s Guide to "Maximum Difference by Remapping a Digit" – LeetCode 2566 (C++ | JavaScript | Python)

Hey Devs! 👋

Let’s break down a fun greedy + string manipulation problem — 2566: Maximum Difference by Remapping a Digit. This one is perfect for sharpening your skills in character replacement and greedy thinking.

📝 Problem Statement

Given a number num, Bob is allowed to remap exactly one digit (0–9) to any other digit (including itself). All occurrences of the selected digit are changed.

Your job:
Return the maximum difference between the largest and smallest number Bob can create by remapping one digit.

✅ Leading zeroes are allowed after remapping.

🧩 Example

Input: num = 11891
Output: 99009

Explanation:

• For maximum value: Replace '1' → '9' → becomes 99899
• For minimum value: Replace '1' → '0' → becomes 890
• Difference = 99899 - 890 = 99009

💡 Strategy & Intuition

To get:

• Maximum value: Change the first non-'9' digit to '9'
• Minimum value: Change the first digit to '0'

Why?

• Changing leftmost significant digits impacts the number most.
• Changing the same digit in all its occurrences ensures a larger shift.

⚙️ C++ Implementation

class Solution {
 public:
  int minMaxDifference(int num) {
    const string s = to_string(num);
    const char to9 = s[firstNotNineIndex(s)];
    const char to0 = s[0];
    return getMax(s, to9) - getMin(s, to0);
  }

 private:
  int firstNotNineIndex(const string& s) {
    for (int i = 0; i < s.length(); ++i)
      if (s[i] != '9')
        return i;
    return 0;
  }

  int getMax(string s, char to9) {
    for (char& c : s)
      if (c == to9)
        c = '9';
    return stoi(s);
  }

  int getMin(string s, char to0) {
    for (char& c : s)
      if (c == to0)
        c = '0';
    return stoi(s);
  }
};

🌐 JavaScript Version

var minMaxDifference = function(num) {
    let s = num.toString();
    const to9 = [...s].find(c => c !== '9') || s[0];
    const to0 = s[0];

    const getMax = s => parseInt(s.replaceAll(to9, '9'));
    const getMin = s => parseInt(s.replaceAll(to0, '0'));

    return getMax(s) - getMin(s);
};

🐍 Python Version

def minMaxDifference(num: int) -> int:
    s = str(num)
    to9 = next((c for c in s if c != '9'), s[0])
    to0 = s[0]

    max_val = int(s.replace(to9, '9'))
    min_val = int(s.replace(to0, '0'))

    return max_val - min_val

🧪 Test Cases

Input: num = 11891
Output: 99009

Input: num = 90
Output: 99

Input: num = 999
Output: 999 - 0 = 999

Input: num = 1000
Output: 9000 - 0 = 9000

⏱️ Time & Space Complexity

Time Complexity: O(n) — One pass for replacement (n = number of digits)
Space Complexity: O(1) — Only a few variables used

✅ Conclusion

This problem is an excellent exercise in greedy digit manipulation and reinforces string traversal techniques.

Remember:

• Maximize by turning first non-9 into 9
• Minimize by turning first digit into 0

Drop a ❤️ if this helped, and stay tuned for more bitesize breakdowns!

Happy coding, folks! 🚀